Ed, USA asked:
Why canít mosquitoes transmit diseases like dirty needles can?
Chris - The thing we know mosquitoes are very good at transmitting is malaria. The way in which they transmit malaria is that people who have malaria are made more susceptible and theyíre more attractive to mosquitoes in the first place. The mosquito flies in, bites the person who is infectious for malaria. The malaria parasite Ė plasmodium Ė then gets inside the body of the mosquito where it infects the gut cells of the mosquito and grows or replicates in there. This means that from a very small infected dose you get a very big dose inside the mosquito. The mosquito then goes and feeds on a second human who isnít infected with malaria. In the course of feeding the mosquito injects some of its saliva into the person which acts as an anticoagulant and also as an immunosuppressant so the personís immune system doesnít attack the mosquito while itís feeding. In the course of injecting that saliva it now injects some of those malarial parasites which are inside it, now at very high levels, into the person. Thatís how the person catches malaria.
Kat - What about things like viruses and HIV?
Chris - With HIV the problem is that itís not infectious but when it comes to a mosquito the problem for HIV is that it canít infect the mosquito. A person who has HIV doesnít actually have very much virus going around in the bloodstream until the latter stages of the disease when theyíve got AIDS or right at the very first part of the disease. The amount of virus taken into the mosquito is very low. When the virus gets into the mosquito it gets broken down or destroyed by the mosquitoís digestive juices. Itís not adapted to infecting or persisting inside the mosquito. When that mosquito bites another person the amount of virus thatís in there is vanishingly small, if not zero. Therefore the amount that gets put back into the next person that gets bitten is absolutely tiny. As a result the next person doesnít get infected. Thatís very useful and very fortunate because with 30 million people on Earth (or more) infected with HIV, if it could spread that way, it would be a major problem. Malaria affects about 3-5 hundred million people every year.